Fuel cells electrochemically convert fuels and oxidants to electricity. Fuel cells can be categorized according to the type of electrolyte (e.g., solid oxide, molten carbonate, alkaline, phosphoric acid, or solid polymer) used to accommodate ion transfer during operation. Moreover, fuel cell assemblies can be employed in many (e.g., automotive to aerospace to industrial) environments, for multiple applications.
A Proton Exchange Membrane (hereinafter "PEM") fuel cell converts the chemical energy of fuels such as hydrogen and oxidant such as air/oxygen directly into electrical energy. The PEM is a solid polymer electrolyte that permits the passage of protons (i.e., H.sup.+ ions) from the "anode" side of a fuel cell to the "cathode" side of the fuel cell while preventing passage therethrough of reactant fluids (e.g., hydrogen and air/oxygen gases). Some artisans consider the acronym "PEM" to represent "Polymer Electrolyte Membrane." The direction, from anode to cathode, of flow of protons serves as the basis for labeling an "anode" side and a "cathode" side of every layer in the fuel cell, and in the fuel cell assembly or stack.
Usually, an individual PEM-type fuel cell has multiple, generally transversely extending layers assembled in a longitudinal direction. In a typical fuel cell assembly or stack, all layers which extend to the periphery of the fuel cells have holes therethrough for alignment and formation of fluid manifolds that generally service fluids for the stack. Typically, gaskets seal these holes and cooperate with the longitudinal extents of the layers for completion of the fluid supply manifolds. As is known in the art, some of the fluid supply manifolds distribute fuel (e.g., hydrogen) and oxidant (e.g., air/oxygen) to, and remove unused fuel and oxidant as well as product water from, fluid flow plates which serve as flow field plates of each fuel cell. Also, other fluid supply manifolds circulate coolant (e.g., water) for cooling.
The PEM can be made using, for instance, a polymer such as the material manufactured by E. I. Du Pont De Nemours Company and sold under the trademark NAFION.RTM.. Further, an active electrolyte such as sulfonic acid groups is included in this polymer. In addition, the PEM is available as a product manufactured by W.L. Gore & Associates (Elkton, Md.) and sold under the trademark GORE-SELECT.RTM.. Moreover, a catalyst such as platinum which facilitates chemical reactions is applied to each side of the PEM. This unit is commonly referred to as a membrane electrode assembly (hereinafter "MEA"). The MEA is available as a product manufactured by W.L. Gore & Associates and sold under the trade designation PRIMEA 5510-HS.
In a typical PEM-type fuel cell, the MEA is sandwiched between "anode" and "cathode" gas diffusion layers (hereinafter "GDLs") that can be formed from a resilient and conductive material such as carbon fabric or paper. The anode and cathode GDLs serve as electrochemical conductors between catalyzed sites of the PEM and the fuel (e.g., hydrogen) and oxidant (e.g., air/oxygen) which flow in respective "anode" and "cathode" flow channels of respective flow field plates.
A given fluid flow plate can be formed from a conductive material such as graphite. Flow channels are typically formed on one or more faces of the fluid flow plate by machining. As is known in the art, a particular fluid flow plate may be a bipolar, monopolar, anode cooler, cathode cooler, or cooling plate.
Flow field plates are commonly produced by any of a variety of processes. One plate construction technique, which may be referred to as "monolithic" style, compresses carbon powder into a coherent mass. Next, the coherent mass is subjected to high temperature processes which bind the carbon particles together, and convert a portion of the mass into graphite for improved electrical conductivity. Then, the mass is cut into slices, which are formed into the flow field plates. Usually, each flow field plate is subjected to a sealing process (e.g., resin impregnation) in order to decrease gas permeation therethrough and reduce the risk of uncontrolled reactions. Typically, flow field channels are engraved or milled into a face of the rigid, resin impregnated graphite plate.
As is well-known in the art, the PEM can work more effectively if it is wet. Conversely, once any area of the PEM dries out, the fuel cell does not generate any product water in that area because the electrochemical reaction there stops. Undesirably, this drying out can progressively march across the PEM until the fuel cell fails completely.
Attempts have been made to introduce water into the PEM by raising the humidity of the incoming reactant gases. That is, the fuel and oxidant gases are often humidified with water vapor before entering the fluid supply manifolds in order to convey water vapor for humidification of the PEM of the fuel cell.
For example, humidification of reactant gases (e.g., fuel and oxidant) is typically attempted by preconditioning the reactant gases at or before introduction of the reactant gases to the flow channels in a fluid flow plate. One method uses externally produced saturated air or hydrogen. Another method uses water injection at the start of each flow channel. Attempts to humidify or pre-mix the correct amount of water to reactant gas are problematic due to the following:
1. The water requirements are not constant from the start of a flow channel to the end of the flow channel; PA1 2. Injecting large amounts of water in order to provide sufficient water and/or humidification at the end of the flow channel often creates one or more cold, wet spots in the cell adding to non-uniform operating temperature distributions and cell performance; PA1 3. Injecting a set amount of water for the entire channel length at the start is often too much for the first quadrant, and too little further downstream; PA1 4. Water requirements across the fuel cell, and along the length of a fuel cell stack, are not uniform but are dynamic and related to cell current densities; PA1 5. Excess water may lead to localized flooding; and PA1 6. Channel dimensions are too small for effective atomization of injected water.
Problems also result from the use of water vapor in humidification of the reactant gases. For example, significant quantities of heat are required in order to saturate a reactant gas at a temperature close to the temperature of the fuel cell. In particular, waste heat from a cell cooling circuit is not sufficient, because the temperature will necessarily be lower than the cell temperature. Furthermore, temperature variations within the reactant gas supply manifolds and fuel cell plate channels can undesirably lead to condensation of the vapor and poor distribution of the reactant gas and vapor/water.
Deleterious effects can also result from turns in the flow path of a stream which is a mixture of water droplets and reactant gas (e.g., two-phase flow). After the stream goes around a given curve, separation of the water from the reactant gas occurs. Anytime the stream changes direction and/or velocity, the various settling rates yield separation. Therefore, by the time the stream reaches the end of such a flow path, most of the liquid water will have settled out. Similar problems and unpredictability can result in any unconstrained flow of water mixed with reactant gas.
Naturally, fuel cells within the same assembly or stack can have varying efficiencies. In particular, some fuel cells generate more heat than others. A fuel cell running hot will require more water in order to function. If a fuel cell assembly delivers inadequate moisture to a given fuel cell, then the PEM of that fuel cell begins to dry out, which causes it to run hotter since the remaining fuel cells in the assembly continue to force high current therethrough. When the PEM of a fuel cell completely dries out, that fuel cell begins to dry out adjacent fuel cells. Accordingly, it is desirable to deliver adequate water to all the fuel cells in the stack.
Additional problems stem from height variations in different areas of an individual fuel cell and the fuel cell assembly, e.g., a fuel cell assembly disposed on an angle and sloping upward from an entry end of a longitudinal reactant fluid supply manifold. The injection of water at the entry of the fluid supply manifold into the fuel cell assembly undesirably results in fuel cells on the low end receiving all water and no gas ("PEM flooding"), and fuel cells on the high end receiving all gas and no water ("PEM starvation").
U.S. Pat. No. 4,973,530 to Vanderborgh et al., entitled "Fuel Cell Water Transport," discloses a serpentine flow field plate for repeatedly passing a fuel cell gas from a reactant flow field to a water transport field, and a membrane plate with a support frame for holding and separating an ion exchange membrane and a water transport membrane. The water transport membrane spans across top portions of the flow channels to define the water transport field. Liquid water on a first face of the water transport membrane is transported across the membrane to an opposite face to control the moisture content of the reactant gas. The water transport system disclosed in Vanderborgh et al. increases the cross-sectional dimensions a fuel cell assembly and increases the thickness of each fuel cell which reduces the overall current density of the fuel cell assembly.
Therefore, there exists a need for compact fluid flow plates which provide substantially uniform and sufficient membrane hydration and/or cooling.